The present invention relates to a signal processing method and a signal processing apparatus for use in such systems as communications equipment, sonar systems, ultrasonic diagnostic apparatus and ultrasonic flaw detectors. The invention also pertains to scanning sonars as well as to a sea bottom mapping sonar employing a crossed fan beam technique. A scanning sonar scans a wide range of directions by forming receiving beams in sequentially varying directions
The present invention will be explained hereinafter as embodied in a sea bottom mapping sonar which is a crossed fan beam type sonar system as a typical example of application of the invention.
A conventionally known crossed fan beam type bottom mapping sonar forms desired receiving beams by phasing echo signals individually received by 160 transducer elements by specific amounts and then combining the phased signals through an analog operation by using 160 mixers, for example.
Since the conventional bottom mapping sonar comprises a large number of transducer elements and receiving channels in its input stage, it has such problems that this sonar system inevitably becomes large-sized and its potential to achieve a high range resolution is more or less limited due to differences in the characteristics of analog circuits or their deterioration with the lapse of time.
An object of the present invention is to provide a signal processing method and a signal processing apparatus applicable to such systems as communications equipment, sonar systems, ultrasonic diagnostic apparatus and ultrasonic flaw detectors whose circuit configuration is simplified by a time-division multiplexing technique.
Another object of the invention is to provide a crossed fan beam type bottom mapping or detecting sonar system using the signal processing method and the signal processing apparatus of the invention.
Another object of the invention is to provide a sonar system, in particular a scanning sonar, which scans a wide range of directions by forming receiving beams in sequentially varying directions using the signal processing method and the signal processing apparatus of the invention.
In one aspect of the invention, a signal processing method comprises the steps of receiving signals of a specific frequency f which is equal to 1/T where T is the period of the signals, sampling the signals at a specific first sampling time instant and at a sampling time instant (a+1/4)T after the first sampling time instant where a is 0 or an integer multiple of 0.5, and outputting data sampled at the sampling time instants as in-phase data and quadrature data of complex-valued sample data.
Preferably, the time interval of (a+1/4)T is smaller than half the recurrence interval of a point of the specific first sampling time instant.
In another aspect of the invention, a signal processing method comprises the steps of receiving signals of a specific frequency f which is equal to 1/T where T is the period of the signals, sampling the signals at a specific first sampling time instant, at a sampling time instant (n+1/4)T after the first sampling time instant, at a sampling time instant (n+1/2)T after the first sampling time instant and at a sampling time instant (n+3/4)T after the first sampling time instant to produce 0xc2x0 sample data, 90xc2x0 sample data, 180xc2x0 sample data and 270xc2x0 sample data respectively, where n is 0 or a positive integer, and outputting a value obtained by averaging the 0xc2x0 sample data and the 180xc2x0 sample data as in-phase data of complex-valued sample data and a value obtained by averaging the 90xc2x0 sample data and the 270xc2x0 sample data as quadrature data of the complex-valued sample data.
Preferably, the time interval of (n+1/4)T is smaller than one quarter of the recurrence interval of a point of the specific first sampling time instant.
In another aspect of the invention, a signal processing device comprises a plurality of signal input means, a plurality of multiplexers which multiplex signals entered from the signal input means into a smaller number of channels having output terminals than the number of the signal input means, wherein the multiplexers operate with synchronized switching timing, and a plurality of A/D converters respectively connected to the output terminals of the multiplexers which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing.
In still another aspect of the invention, a signal processing device comprises a plurality of signal input means for inputting signals having a specific frequency, a plurality of multiplexers which multiplex the signals entered from the signal input means into a smaller number of channels having output terminals than the number of the signal input means, wherein the multiplexers operate with synchronized switching timing, a plurality of A/D converters respectively connected to the output terminals of the multiplexers which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing, and a phase shifter which shifts the phases of individual sample data such that a specific phase relationship is established between the data sampled by the A/D converters.
In yet another aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed successively and oriented in successively varying directions comprises a plurality of transducer elements for receiving echo signals of a specific frequency f which is equal to 1/T where T is the period of the echo signals, a plurality of multiplexers which multiplex signals entered from the transducer elements into a smaller number of channels than the number of the transducer elements, wherein the multiplexers operate with synchronized switching timing, a plurality of A/D converters which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing and sample the analog signals at a specific first sampling time instant, at a sampling time instant (n+1/4)T after the first sampling time instant, at a sampling time instant (n+1/2)T after the first sampling time instant and at a sampling time instant (n+3/4)T after the first sampling time instant to produce 0xc2x0 sample data, 90xc2x0 sample data, 180xc2x0 sample data and 270xc2x0 sample data respectively, where n is 0 or a positive integer, outputting a value obtained by averaging the 0xc2x0 sample data and the 180xc2x0 sample data as in-phase data of complex-valued sample data and a value obtained by averaging the 90xc2x0 sample data and the 270xc2x0 sample data as quadrature data of the complex-valued sample data, a phase shifter shifts the phases of individual sample data such that a specific phase relationship is established between the complex-valued sample data derived from the individual transducer elements, and a matched filter for receiving the complex-valued sample data from the phase shifter and successively forming the receiving beams in different directions.
In yet another aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed successively and oriented in successively varying directions comprises a plurality of transducer elements for receiving echo signals of a specific frequency, a plurality of multiplexers which multiplex signals entered from the transducer elements into a smaller number of channels than the number of the transducer elements, wherein the multiplexers operate with synchronized switching timing, a plurality of A/D converters which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing and sample the analog signals for a plurality of waves thereof from the transducer elements to produce 0xc2x0 sample data, 90xc2x0 sample data, 180xc2x0 sample data and 270xc2x0 sample data of echo signals from each of the transducer elements respectively, outputting a value obtained by averaging the 0xc2x0 sample data and the 180xc2x0 sample data as in-phase data of complex-valued sample data and a value obtained by averaging the 90xc2x0 sample data and the 270xc2x0 sample data as quadrature data of the complex-valued sample data, a phase shifter shifts the phases of individual sample data such that a specific phase relationship is established between the complex-valued sample data derived from the individual transducer elements, and a matched filter for receiving the complex-valued sample data from the phase shifter and successively forming the receiving beams in different directions.
In yet another aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed successively and oriented in successively varying directions comprises a plurality of transducer elements for receiving echo signals of a specific frequency, a plurality of multiplexers which multiplex signals entered from the transducer elements into a smaller number of channels than the number of the transducer elements, a plurality of A/D converters which convert analog signals entered respectively and individually from the multiplexers into digital form, means for successively generating in a sequential order in-phase data of complex-valued sample data and quadrature data of complex-valued sample data from the digital signals, and a matched filter for receiving the complex-valued sample data from the generating means and successively forming the receiving beams in different directions.
In a further aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed successively and oriented in successively varying directions comprises a plurality of groups of transducer elements for receiving echo signals of a specific frequency f which is equal to 1/T where T is the period of the echo signals, a plurality of multiplexers which multiplex signals entered successively from each of the groups of the transducer elements into a smaller number of channels than the number of the transducer elements, wherein the multiplexers operate with synchronized switching timing, a plurality of A/D converters which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing and repeatedly sample the analog signals from the each of the groups of the transducer elelments at a specific first sampling time instant, at a sampling time instant 1/4xc2x7T after the first sampling time instant, at a sampling time instant 1/2xc2x7T after the first sampling time instant and at a sampling time instant 3/4xc2x7T after the first sampling time instant to produce 0xc2x0 sample data, 90xc2x0 sample data, 180xc2x0 sample data and 270xc2x0 sample data respectively, outputting a value obtained by averaging the 0xc2x0 sample data and the 180xc2x0 sample data as in-phase data of complex-valued sample data and a value obtained by averaging the 90xc2x0 sample data and 270xc2x0 sample data as quadrature data of the complex-valued sample data, a phase shifter shifts the phases of individual sample data such that a specific phase relationship is established between the complex-valued sample data derived from the individual transducer elements, and a matched filter for receiving the complex-valued sample data from the phase shifter and successively forming the receiving beams in different directions.
In a still further aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed successively and oriented in successively varying directions comprises a plurality of groups of transducer elements for receiving echo signals of a specific frequency, a plurality of multiplexers which multiplex signals entered successively from each of the groups of the transducer elements into a smaller number of channels than the number of the transducer elements, wherein the multiplexers operate with synchronized switching timing, a plurality of A/D converters which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing, means for generating in-phase data of complex-valued sample data and quadrature data of complex-valued sample data from the digital signals, and a matched filter for receiving the inphase data of complex-valued sample data and quadrature data from the generating means and successively forming the receiving beams in different directions.
In a still further aspect of the invention, a sonar system which emits an ultrasonic search pulse signal and receives echo signals by receiving beams formed and oriented in varying directions comprises a plurality of transducer elements, a plurality of multiplexers which multiplex signals supplied from the transducer elements into a smaller number of channels having output terminals than the number of the transducer elements, wherein the multiplexers operate with synchronized switching timing, and a plurality of A/D converters respectively connected to the output terminals of the multiplexers which convert analog signals entered respectively and individually from the multiplexers into digital form, wherein the A/D converters operate with synchronized sampling timing.
In sonar systems like a bottom mapping sonar, the frequency of echo signals is almost same as the transmitting frequency because the Doppler effect is substantially negligible. Therefore, even if the echo signals are not sampled to obtain I- and Q-signals at the same time, it is possible to obtain in-phase data and quadrapture data by sampling the echo signals twice with a phase delay of 90xc2x0, or with a time delay of (a+1/4)T. According to this time-division method, it is possible to produce the in-phase data and quadrapture data without the provision of two channels for the in-phase data and quadrapture data.
The time interval between sampling of the in-phase data and sampling of the quadrapture data in one sampling cycle is made smaller than the time interval between sampling of the in-phase data in one sampling cycle and that in a succeeding sampling cycle. More specifically, the interval between in-phase data and quadrapture data sampling times in one sampling cycle is made smaller than half the recurrence time interval of successive sampling cycles as shown in FIG. 10A. As a consequence, it is possible to obtain complex-valued sample data with minimal errors even when the amplitude of an input echo signal varies as shown in FIG. 10B or its frequency deviates.
The smaller the time interval between sampling of in-phase data and sampling of quadrapture data, the better the results obtained. Optimum results are obtained when a=0, or when the time interval is T/4. Shown in FIG. 10B is an example in which the phase of the sample data is 45xc2x0. In this example, the sample value of in-phase data obtained at a sampling time to is 0.4861359 and the sample value of in-phase data obtained at a sampling time ti which is delayed by T/4 from to is 0.5155987. The phase obtained from these values is 43.32xc2x0, which contains an error equivalent to 0.0026 times the wavelength. Contrary to this, when the in-phase data and quadrapture data are sampled at equally-spaced time intervals (1.25T in the example of FIG. 10B), the sample value obtained at a sampling time t1 which is delayed by 1.25T from to is 0.6334498, and the phase obtained from this sample value and the sample value for obtained at the sampling time to is 37.50xc2x0, representing an error equivalent to 0.021 times the wavelength. It is understood from above that if the interval between in-phase data and quadrapture data sampling times in one sampling cycle is made smaller than half the recurrence time interval of successive sampling cycles, it is possible to obtain complex-valued sample data with minimal errors even when the amplitude or frequency of input signals varies or deviates.
It is also possible to cancel out a direct-current (DC) bias component superimposed on sample data by performing an averaging operation using sample data obtained at sampling times (n+1/2)T and (n+3/4)T.
When signals entered from a plurality of signal input means are multiplexed by using multiplexers, there can arise a case where noise produced by switching of the multiplexer in one channel adversely affects sample data in another channel if the multiplexers are switched one after another at regular time intervals. To avoid this problem, switching timing of all the multiplexers and sampling timing of all the A/D converters synchronized in this invention such that noise would not be induced into the sample data.
In a bottom mapping sonar, for example, signals entered from multiple channels are sequentially sampled at regular time intervals and resultant data which are arranged obliquely to a time axis are entered to a matched filter to thereby scan a sea bottom. Even when the input signals are sampled with synchronized timing as mentioned above, the phase of the sample data is shifted in such a way that a data string arranged obliquely to the time axis would be obtained. Alternatively, data sampled in a steplike form may be shifted such that data sampled at the same time would be obtained.
As stated above, data is sampled twice with a phase delay of (n+1/4)T to obtain in-phase data and quadrapture data in this invention. Therefore, it is possible to produce complex-valued sample data without increasing the number of A/D converters or mixers.
According to the invention, even when DC bias components are superimposed on the sample data, it is possible to remove them by using 180xc2x0 sample data and 270xc2x0 sample data.
When the multiplexer of one channel is switched, it usually produces electrical noise which would adversely affect sample data of other channels. With this invention, however, data is not sampled in any channel when noise is generated, and the multiplexer is not switched in any channel during sampling process, because all the multiplexers are switched at the same time. This makes it possible to eliminates the influence of the noise. Furthermore, it is possible to achieve successive beamforming by use of a matched filter, for instance, by shifting sampling times arranged in a steplike form to a horizontal-line (simultaneous) sampling scheme or to an oblique-line sampling scheme.